Coccidiosis is a disease of both invertebrates and vertebrates, including man, caused by intracellular parasitic protozoa which generally invade the epithelial cells lining the alimentary tract and the cells of associated glands. The crowded conditions under which many domestic animals are raised have contributed to increased incidence of the disease. Virtually every domestic animal is susceptible to infection, and distribution of the parasite is world-wide. Coccidiosis is therefore the cause of significant economic loss throughout the world.
The poultry industry suffers particularly severe losses, with coccidiosis being the most economically important parasitic disease of chickens. Since 1949, preventive anticoccidials have been used but have not been totally effective. Losses due to morbidity from coccidiosis, including reduced weight gains and egg production, and decreased feed conversion, persist. The cost of coccidiosis in broiler production has been estimated at 1/2 to 1 cent per pound. Based on an annual production of 3,000,000,000 broilers annually in the United States, losses would total between 60 and 120 million dollars. To this figure must be added the cost of anticoccidials estimated at 35 million dollars. These impressive figures emphasize the importance of reducing the incidence of coccidiosis in chickens.
Of the nine genera of coccidia known to infect birds, the genus Eimeria contains the most economically important species. Various species of Eimeria infect a wide range of hosts, including mammals, but nine species have been recognized as being pathogenic to varying degrees in chickens: Eimeria acervulina, E. mivati, E. mitis, E. praecox, E. hagani, E. necatrix, E. maxima, E. brunetti and E. tenella.
Although the Eimeria are highly host specific, their life cycles are similar. The developmental stages of the avian coccidia can be illustrated by the species Eimeria tenella, which proliferates in the cecum of the chicken.
The life cycle of the Eimeria species begins when the host ingests previously sporulated oocysts during ground feeding or by inhalation of dust. Mechanical and chemical action in the gizzard and intestinal tract of the chicken ruptures the sporulated oocyst, liberating eight sporozoites. The sporozoites are carried in the digestive contents and infect various portions of the intestinal tract by penetration of epithelial cells. Subsequent life stages involve asexual multiple fission, the infection of other epithelial cells, development of gametes, and fertilization to produce a zygote which becomes an oocyst which is passed out of the host with the droppings. The oocyst undergoes nuclear and cellular division resulting in the formation of sporozoites, with sporulation being dependent upon environmental conditions. Ingestion of the sporulated oocyst by a new host transmits the disease.
Of all species of Eimeria, E. tenella has received the most attention. E. tenella is an extremely pathogenic species, with death often occurring on the fifth or sixth day of infection.
Before the use of chemotherapeutic agents, poultry producers' attempts to control coccidiosis were limited to various management programs. These programs were directed toward attempts at sanitation through disinfection, or by mechanical removal of litter. Despite these efforts, sufficient oocysts usually remained to transmit the disease.
One means of combating the hazards of coccidia is immunization. This method involves feeding to the poultry a small dose of oocysts of each of the species of coccidia during the first weeks of life. However, dosage control has proven difficult as birds ingest daughter oocysts, with some birds developing severe coccidiosis and others remaining susceptible. Also, since this procedure produces mixed infections, sometimes adequate immunity does not develop to all species given. In addition, immunity development is too slow for use with broiler production.
Another means of combating coccidia is drug treatment after the poultry is infected. One drug that has been used is sulfanilamide which has shown anticoccidial activity against six species of coccidia. However, unless the drug treatment of the flock is quickly initiated after diagnosis of the disease, medication may be started too late to be effective.
Ideally, the best method for combating coccidia is preventive medication. Since the advent of the use of sulfonamide drugs, over forty compounds have been marketed for preventive medication against coccidia. There have been many problems with the use of such drugs, including anticoccidial contamination of layer flock feeds, inclusion of excessive anticoccidial drugs in the feed causing toxicity in the birds and omission of the anticoccidial from the feed resulting in coccidiosis outbreaks. A particularly frustrating problem has been the development of drug-resistant strains of coccidia. Moreover, there is a potential for drug residues being deposited in the meat.
Clearly, available methods for the control of coccidiosis have met with limited success, and the need for a safe, efficient, and inexpensive method of combating avian coccidiosis remains.
The development of an effective anticoccidial vaccine is a desirable solution to the problem of disease prevention. Vaccines produced by traditional methods will require extensive development. There are reports of the production of attenuated strains through passage in embryos or cell culture. While this approach may eventually lead to successful vaccines, not all the important species of Eimeria have been adapted to growth in culture or embryos such that they are capable of completing their life cycle.
Genetic engineering methodology presents the opportunity for an alternative approach to vaccine development. It is known that genes encoding antigenic proteins of pathogenic organisms can be cloned into microorganisms. The antigenic proteins then can be expressed at high levels, purified, and used as vaccines against the pathogenic organism. These antigenic proteins have the advantage of being noninfectious and are potentially inexpensive to produce. Such "subunit vaccines" have been prepared from antigen genes for a number of viruses such as hepatitis, herpes simplex and foot and mouth disease virus. An alternate approach is to produce "synthetic vaccines", small chemically-synthesized peptides, whose sequence is chosen based upon the amino acid sequence translation of viral antigen DNA. The advantages of such "synthetic vaccines" over traditional vaccination with attenuated or killed pathogenic organisms have been summarized by Lerner in Nature 299:592-596 (1982).
It is now possible to produce foreign proteins, including eukaryotic proteins, in prokaryotic organisms such as gram positive or gram negative bacteria. The process involves the insertion of DNA (derived either from enzymatic digestion of cellular DNA or by reverse transcription of mRNA) into an expression vector. Such expression vectors are derived from either plasmids or bacteriophage and contain: (1) an origin of replication functional in a microbial host cell; (2) genes encoding selectable markers, and (3) regulatory sequences including a promoter, operator, and a ribosome binding site which are functional in a microbial host cell and which direct the transcription and translation of foreign DNA inserted downstream from the regulatory sequences. To increase protein production and stability, eukaryotic proteins are often produced in prokaryotic cells as a fusion with sequences from the amino-terminus of a prokaryotic protein. .beta.-Galactosidase or the product of one of the E. coli tryptophan operon genes have been used successfully in this manner. Expression vectors have also been developed for expression of foreign proteins in eukaryotic host cells, e.g., yeast and chinese hamster ovary tissue culture cells.
Host cells transformed with expression vectors carrying foreign genes are grown in culture under conditions known to stimulate production of the foreign protein in the particular vector. Such host cell/expression vector systems are often engineered so that expression of the foreign protein may be regulated by chemical or temperature induction. Proteins which are secreted may be isolated from the growth media, while intracellular proteins may be isolated by harvesting and lysing the cells and separating the intracellular components. In this manner, it is possible to produce comparatively large amounts of proteins that are otherwise difficult to purify from native sources.
Such microbially produced proteins may be characterized by many well-known methods, including the use of monoclonal antibodies, hereinafter referred to as "MCAs," which are homogeneous antibodies that react specifically with a single antigenic determinant and display a constant affinity for that determinant, or by use of polyvalent antibodies derived from infected birds, which react with a variety of different antigens and often with multiple determinants on a single antigen.
Alternate technology to the production of "subunit" or "synthetic" vaccines is the use of a fowl pox virus vector. The pox virus vaccinia has a long history of use as a vaccine and has been employed to virtually irradicate smallpox in humans. It now has been demonstrated that vaccinia virus can be effectively genetically engineered to express foreign antigens (Smith et al., Nature 302:490-495 (1983); Panicali et al., Proc. Natl. Acad. Sci. U.S.A. 80:5364-5368 (1983); Mackett et al., J. of Virology 49:857-864 (1984)) and the engineered viruses can serve as a live vaccine against other viruses and infections besides smallpox. Fowl pox virus is very similar to vaccinia virus and many of the methods developed for vaccinia for the creation of recombinants expressing foreign antigens can be applied to fowl pox. Attenuated fowl pox virus engineered to produce avian coccidia antigens thus is another method to produce an anticoccidial vaccine. Live vaccines have the advantage of being inexpensive to produce and are characterized by the production of rapid immunity development.
A second type of live vaccine results in the presentation of antigen in the gut where coccidia normally invades. This method utilizes secretion or outer surface expression of the antigen by harmless bacteria introduced into the intestinal microbial population by incorporation in feed. Secretion is obtained by fusion of an antigen gene to the gene coding for a protein which is normally secreted, leaving the necessary secretion signal sequence intact. Outer surface expression is achieved b fusion of the antigen genes to the genes that code for proteins normally localized on the outer surface. (T. Silhavy, U.S. Pat. No. 4,336,336.) This type of live vaccine is especially advantageous since manufacturing costs are minimal and the immune response stimulated is of a type particularly effective against coccidia invasion of the gut.